The invention relates to the recovery of oil from subterranean formations by surfactant flooding operations. More particularly, the invention concerns the use of a surfactant flooding system wherein the surfactant slug is followed by a low salinity polyacrylamide buffer slug which is followed by a drive fluid containing polysaccharides.
Investigations of ways to increase oil recovery by improving the displacement ability of a water flood have produced useful surfactants which reduce the interfacial tension between the oil and water in the reservoir. With lower interfacial tension, oil that is trapped in the pore structure can disperse into the water as smaller and more easily deformable droplets. Many types of surfactants have been investigated and the choice of which surfactant to employ in a water flood operation is dependent upon the conditions in the reservoir, as well as the cost and availability of the surfactants.
Most water flood operations have employed a petroleum sulfonate as a sole surfactant, or at least a major component of a mixture of surfactants. Synthetic alkylaryl sulfonates and alkyl sulfonates and sulfates have been proposed as oil recovery surfactants. Sulfonates are preferred because they have a better high temperature stability than the sulfates. These surfactants are all classified as organic sulfonates and are usually metal salts of alkylbenzene sulfonate containing 12 to 30 carbon atoms, but may also be aliphatic sulfonates or alkylated naphthalene sulfonates. These surfactants have an equivalent weight that ranges from 320 to 700 g/mole.
To combat layering and precipitation problems in high salinity applications, a material with both water-soluble and oil soluble characteristics is usually added to organic sulfonate surfactant mixtures. When used in surfactant flooding, these materials are generally referred to as "solubilizers" and often constitute the most expensive component in a surfactant mixture. Conventional solubilizers are sulfate or sulfonate salts of polyethoxylated alcohols or alkyl phenols. The amount of solubilizer required depends on the amount of and types of organic sulfonate surfactants employed in the water flood operation. A minimum amount of solubilizer is required to prevent the surfactants from precipitating from the flood water. The choice of the solubilizer employed is dependent on the choice of surfactants to be used and the salinity of the flood water. Surfactant quantity is a function of the reservoir's size and other characteristics. The concentration of surfactant components in the system is usually 1% to 6% expressed on an active surfactant basis.
Generally, a surfactant slug is followed by a polymer drive fluid. It is also known to inject the polymer drive fluid in blocks having varying polymer concentrations and varying viscosities. U.S. Pat. No. 3,467,187 discloses the use of a surfactant displacement slug and a mobility buffer with graded viscosity zones injected between the surfactant and the drive fluid. U.S. Pat. Nos. 3,500,924 and 3,507,331 disclose the use of a mobility buffer fluid between the surfactant slug and the drive fluid wherein the front part of the mobility buffer fluid contains sufficient surfactant to emulsify with the back of the surfactant slug and a viscosity in the rear of the mobility buffer fluid to match up with the viscosity of the front of the drive fluid.
U.S. Pat. Nos. 3,707,187; 4,074,759; 4,099,569 and 4,100,966 all disclose varying the concentration of a surfactant in a polymer slug for mobility control, wherein the surfactant concentration is tapered. U.S. Pat. No. 3,670,818 describes following a surfactant slug with alternate aqueous slugs containing first a mobility reducing agent, then slugs of water, and then a drive fluid. The general idea behind all of these disclosures is to provide for compatible viscosities between adjacent slugs to reduce fingering between slugs and increase recovery efficiency.